Powder mixture for the production of alloy steel with a low content of oxide inclusions

ABSTRACT

A powder mixture for manufacturing of alloy steel wherein the metal powder portion consists of a mixture of two powders, viz. a first atomized prealloyed steel powder and a second non atomized alloy powder comminuted from a solidified melt. The alloying elements are distributed in such a way that elements, the oxides of which are easily reducible preferably nickel, copper, molybdenum and/or cobalt substantially enter into the atomized prealloyed steel powder and the oxidation sensitive elements especially chromium and manganese enter into the finely communited powder.

Unite States atent [1 1 Lindskog et al.

[ Aug. 12, 1975 POWDER MIXTURE FOR THE PRODUCTION OF ALLOY STEEL WITH A LOW CONTENT OF OXIDE INCLUSIONS Inventors: Per Folke Lindskog, Hoganas; Per

Gunnar Arbstedt; Erik Goran Wastenson, both of /iken, all of Sweden Assignee: Hoganas AB, Fack, I-loganas,

Sweden Filed: Oct. 29, 1974 Appl. No.: 518,474

FOREIGN PATENTS OR APPLICATIONS 164,156 10/1949 Austria 75/.5 BA

Primary ExaminerL. Dewayne Rutledge Assistant ExaminerArthur J. Steiner Attorney, Agent, or FirmAlbert L. Jeffers; Roger M. Rickert 5 7 ABSTRACT A powder mixture for manufacturing of alloy steel wherein the metal powder portion consists of a mixture of two powders, viz. a first atomized prealloyed steel powder and a second non atomized alloy powder comminuted from a solidified melt. The alloying elements are distributed in such a way that elements, the oxides of whichare easily reducible preferably nickel, copper, molybdenum and/or cobalt substantially enter into the atomized prealloyed steel powder and the oxidation sensitive elements especially chromium and manganese enter into the finely communited powder.

8 Claims, N0 Drawings POWDER MIXTURE FOR THE PRODUCTION OF ALLOY STEEL WITH A LOW CONTENT OF OXIDE INCLUSIONS BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to metal powders for the manufacture of alloy steel and especially low alloy steel. Such manufacturing can be carried out by means of the conventional powder metallurgical method (pressing and sintering) or by means of the developing hot forging method.

2. Description of the Prior Art In the first ease metal powder is compressed in dies to compacts which are very close to the desired shape of the finished product. The green compact is then subjected to a heat treatment in which the powder particles sinter to each other and the compact obtains a good strength. Products produced in this way have a certain porosity causing a low ductility and toughness. The porosity, which can vary considerably from one material to another. but usually amounts to 7 to 20 per cent by volume. can be eliminated by using the above mentioned hot forging technique, whereby the ductility and toughness can be considerably improved. The most important step in the case of hot forging is the pressing of heated preformed compacts in a die. The result of hot forging is a body with full density.

In the production of steel products from metal powder according to said latter technique. however. oxide impurities present in the powder have a much more damaging influence on the mechanical properties than they have in products compressed and sintered in conventional manner.

Powders for the production of such alloy steel products can be divided into two principally different groups. First. it is possible to mix different powders each consisting of one or more but not all of the alloying elements of the finished product.

Secondly, it is possible to atomize a melt having exactly the composition desired in the end product. Thus. in the latter case each powder particle is homoge neously alloyed with the same proportions of alloying elements as desired in the end product. However, this alloying method has some disadvantages which in some cases make it directly unsuitable.

One of the most serious disadvantages is due to oxide impurities in alloy powders. In the most common and from the economical point of view completely superior of the atomizing methods, viz.. atomization with water, some of the powder particles are oxidized during the atomizing process. In order to reduce the oxides thereby formed the powder is subjected to a subsequent heating in reducing atmosphere, e.g. cracked ammonia.

The heating of green compacts in reducing atmosphere before the forging offers a further possibility of reducing remaining oxides in the powder. since this heating is carried out to a higher temperature than the one used for the annealing of the powder. By means of the present technique it is possible to produce substantially oxide free forged materials provided that the steel powder only contains alloying elements. the oxides of which are relatively easily reduced. i.e. have a free energy of formation with an absolute value lower than 120 kcaI/mole O (502 kJ/mole 0 at IO0U C.,Values of the free energy of formation for oxides of some of the alloying elements most commonly present in steel calculated according to Kubaschewsky, O. & Evan. L. I... Metallurgical Thermochemistry, London I956. are given in the table below.

Oxides having an absolute value of the free energy of formation exceeding 120 kcal/mole O (502 kJ/mole 0 at I000 C, are not at all or incompletely reduced in the present technique. This causes that oxidation sensitive elements, such as Cr and Mn, which from the economical as well as the alloying point of view are very desirable. can be used only to a very limited extent as alloying elements in atomized steel powder. This is due to the fact that they form oxides during atomization. which cannot be eliminated during further processing.

In the end product these oxides then cause a strong impairment of ductility and toughness. These properties. however, are also dependent of the size of the oxide inclusions as shown by experiments. At predetermined total oxygen content in the end product the material obtains considerably impaired mechanical prop erties when the oxygen is present in the form of coarse impurities in comparison with material where the oxide impurities are small but therefore more numerous. This critical size of the oxide inclusions, above which they cause a strong impairment of the properties of the material is between 20 um and 100 am.

The present invention provides for a solution of the problem of avoiding the above mentioned difficulties by means of a suitable powder mixture.

SUMMARY OF THE INVENTION The basic idea of the invention is that the metal powder portion consists of a mixture of two powders. viz.. a first atomized prealloyed steel powder and a second non atomized alloy powder comminuted from a solidified melt. The alloying elements are distributed in such a way that elements. the oxides of which are easily reducible (preferably nickel. copper. molybdenum and- /or cobalt) substantially enter into the atomized prealloyed steel powder and the oxidation sensitive elements (especially chromium and manganese) enter into the finely comminuted powder. The above is not valid for carbon which is added substantially as graphite in quantities up to I71 of the powder mixture or as an alloying element in the finely comminuted alloy powder in a content of 10% at the most of this powder. Here as well as in the continuation of the specification and claims /1 means percent by weight.

DETAILED DESCRIPTION OF THE INVENTION The atomized prealloyed powder is produced by melting iron and ().2l ()7: of alloying elements. the oxides of which have an absolute value of the free energy of formation below 120 kcal/mole O 502 kJ/mole O- at 1000 C. water atomization of the melt and finally annealing in a reducing atmosphere (suitably cracked ammonia). Depending upon the quality of the iron raw material the atomized prealloyed powder can contain up to 0.4% accessory elements. the oxides of which have an absolute value of the free energy of formation above 120 kcal/mole O (502 kJ/mole at 1000 C. The content of such accessory elements. the oxides of which have an absolute value of the free energy of formation above 150 kcal/mole O- (627 kJ/mole O- at 1000 C, should not exceed 0.1%. Particularly the contents of silicon. titanium and aluminum should not exceed 0.04%. 0.03% and 0.03%. respectively. The powder should have such a particle size distribution that more than 90% and preferably more than 97% of the powder passes a sieve having a mesh opening of 175 .1.m (80 Tyler mesh; Method for sieve analysis ofgranu lar metal powders. MPlF Standard 5-46. Metal Powder Industries Federation. New York. USA).

Other alloying elements. the oxides of which have an absolute value of the free energy of formation exceeding 120 kcal/mole O (502 kJ/mole 0 at 1000 C, are molten possibly under addition of at most 75% preferably at most 50% iron and/or other metals with easily re ducible oxides and carbon. and is cast. The ingot is crushed and comminuted to a fine alloy powder. The total content of elements. the oxides of which have an absolute value of the free energy of formation exceeding 150 kcal/mole O (627 kJ/mole 0 at 1000 C. should not be higher than 1% in the finely comminuted alloy powder.

The two components are now mixed in the proportions 80-99% atomized prealloyed powder and l% finely comminuted alloy powder. A powder mixture produced in this way provides a material having small and few oxide particles which is shown by the following example.

EXAMPLE l Two powders were produced. one by atomizing by means of water and subsequent annealing in cracked ammonia (A). the other by mixing finely ground ferro alloy powder and water atomized. molybdenumalloyed steel powder (B). The two powders (A) and (B) had the composition of 1% Mn. 1% Cr. 0.5% Mo. balance Fe. The ferroalloy powder used in powder (B) had the composition Mn. 25% Cr. 7% C. balance Fe. and an average particle size of 4 am according to Fischer (Methods for determination of average particle size of metal powder by the Fischer sub-sieve sizer. MPlF Standard 32-60. Metal Powder Industries Feder ation. New York. USA). In both cases graphite was added in such an amount that the carbon content of the powders amounted to 0.5%.

Green compacts in the form of cylinders with 25 mm diameter and mm length were pressed from both powders. The compacts were heated, one group at 1 120 C in hydrogen gas atmosphere with a dew point of 20 C and maintained at these temperatures for 30 minutes. From the furnace the compacts were ripidly brought to a die where the cylinders at the elevated temperature were compressed to full density.

In order to estimate the content of oxide inclusions the cylinders were analyzed with regard to total oxygen content. Further. the number of oxide inclusions hav- The results are shown in the following table.

Material ()xygcn content Number of oxide inclusions The above example shows the advantage of the present invention. When Cr and Mn. the oxides of which have a free energy of formation of l26 and 140 kcal/mole O (-528 and 586 kJ/mole at 1000 C respectively. were added as a finely comminuted ferroall y (powder B) the forged product obtained a considerably lower oxygen content and lower number of large inclusions than in the case of alloying chromium and manganese already into the atomized steel powder (powder A).

In addition to low content of inclusions it is desired. as mentioned above. that the alloying elements are to a large extent homogeneously distributed in the finished product. For this purpose the component of the mixture containing the oxidation sensitive alloying elements according to the invention must be comminuted to a small particle size. This is illustrated in the followmg example.

EXAMPLE 2 Three powder mixtures were produced. all of them with the composition 1% chromium. 2% nickel and 0.5% molybdenum. balance iron. in all of the mixtures the components consisted of water atomized prealloyed steel powder comprising 2% nickel and 0.5% molybdenum. and ferro-chromium powder comprising chromium and 0.3% carbon. In the first case (C) the ferrochromium powder had an average particle size according to Fischer of 33 ,um. in the second case (D) 20 um and in the third case (E) 4 am. Before compaction 0.5% graphite powder and 0.8% zinc stearate were mixed into the three powders. They were then compacted to cylinders which were heated to 1 C in partially combusted propane with a dew point of 3 C and then forged in the manner described in example 1. The cylinders were cut into halves. and the cut surfaces 50 were ground and polished. The chromium content was measured in several points of the cut surfaces by means of a microprobe. The coefficient of variation (CV) for these data were calculated as the standard deviation in percent of the average concentration. CV is a measure of the heterogeneity of the material. Material made from powder C had the CV-value 175%. For powder D the CV-value was and for powder E 45%. At a heterogeneity corresponding to CV the hardenability enhancing effect of chromium is almost nonexisting which means that the addition of chromium in powder C is of no value whereas powder D and particularly powder E resulted in material in which the hardenability was increased by the addition of chromium.

From the above examples the advantage of adding that component which contains oxidation sensitive alloying elements finely in a comminuted state if obvious. The powder particles should have an average size according to Fischer below 20 um. The example also (all shows that it is still more advantageous that the said average size is less than um.

Of course it is important that the powder described above does not segregate after the mixing operation. Possibilities for segregation occur during transporta tion of the powder from the mixer to the consumer and during feeding to the powder press. One way of dimish ing the risk of segregation is to add 5()2()() g of a light mineral oil per metric ton powder continuously during the filling of the mixer. Thereby the fine constituents are brought to stick to the coarser steel particles.

A further improvement in this respect is obtained if the mixture is subjected to a heat treatment at 65()900 C for a period of minutes to 2 hours in reducing atmosphere with subsequent cautious disintegration of the cake formed. By this treatment the finely comminuted alloy powder particles are sintercd to the steel powder particles which effectively counteracts segregation. This cautious sintering treatment can be carried out on powder to which the above mentioned oil has been added as well as on powder without the addition of such oil.

In such cases where the powder mixture has a low content of finely comminuted alloy powder. this finely comminuted powder can advantageously be mixed with only part of the steel powder to form a concentrate. This concentrate is then subjected to one of the above described processes for diminishing the risk of segregation. Finally this concentrate is mixed with such a quantity of steel powder that the desired composition is obtained.

in addition to the components mentioned the mixtures according to the above specification can contain a suitable lubricant. c.g. zinc stearate. The addition of lubricant should not exceed 1%.

We claim:

1. A powder mixture for manufacturing of alloy steel articles having small and few oxide inclusions. comprising a metal powder portion which consists of a mixture of two powders. viz. an atomized prealloyed steel powder and a finely comminuted powder containing alloying elements. wherein alloying elements. the oxides of which have a free energy of formation with an absolute value less than 120 kcal/mole O (502 kJ/molc 0 at 1()O() C substantially are contained in the atomized prealloyed powder while all alloying elements. the oxides of which have a free energy of formation with an absolute value exceeding 120 kcal/mole O (502 kJ/mole 0 at 1()0O C. are completely contained in the finely comminuted powder.

2. A powder mixture according to claim 1. consisting of to 99% of an atomized prealloyed powder of iron and (1.2 to 10% of said alloying elements, the oxides of which have a free energy of formation with an absolute value of less than kcal/mole O (502 kJ/mole 0 at 10()() C. and ofa finely comminuted powder consisting of a total of at least 25% and preferably at least 50% of said alloying elements. the oxides of which have a free energy of formation with an absolute value exceeding 120 kcal/mole O (502 kJ/mole 0 at 1000 C. at most 10% carbon. the rest iron and/or other alloying elements. the oxides of which have a free energy of formation with the absolute value lower than 120 kcal/- mole O (502 kJ/mole 0 at 1000 C.

3. A powder mixture according to claim 1. in which the alloying elements contained in the atomized prealloyed powder consist of one or more of the elements nickel. copper. molybdenum and cobalt.

4. A powder mixture according to claim 1, in which the alloying elements of the finely comminuted powder are one or both of the elements chromium and manganese.

5. A powder mixture according to claim I, in which that the finely comminuted powder consists of a ferro alloy with a total of at least 25% and preferably at least 50% of said alloying elements. the oxides of which have a free energy of formation with an absolute value exceeding 120 kcal/mole O (502 kJ/mole 0 at 1()()() C. at most 10% of carbon and the balance iron.

6. A powder mixture according to claim 1, in which the finely comminuted powder has an average particle size according to Fischer of at most 20 um and preferably at most 5 ,um.

7. A powder mixture according to claim 1, in which the particle size distribution of the atomized prealloyed powder is such that more than 90% and preferably more than 97% of the powder passes a sieve with the mesh opening of ,um.

8. A powder mixture according to claim 1, in which the mixture contains in addition to the metal powder portion graphite and/or a lubricant such as zinc stearate up to a maximum 1% of each. 

1. A POWDER MIXTURE FOR MANUFACTURING OF ALLOY STEEL ARTICLES HAVING SMALL AND FEW OXIDE INCLUSIONS, COMPRISING A METAL POWDER PORTION WHICH CONSISTS OF A MIXTURE OF TWO POWDERS, VIZ. AN ATOMIZED PREALLOYED STEEL POWDER AND A FINELY COMMINUTED POWDER CONTAINING ALLOYING ELEMENTS, WHEREIN ALLOYING ELEMENTS, THE OXIDES OF WHICH HAVE A FREE ENERGY OF FORMATION WITH AN ABSOLUTE VALUE LESS THAN 120 KEAL/MOLE O2 (502 KJ/MOLE O2) AT 1000*C SUBSTANTIALLY ARE CONTAINED IN THE ATOMIZED PREALLOYED POWDER WHILE ALL ALLOYING ELEMENTS, THE OXIDES OF WHICH HAVE A FREE ENERGY OF FORMATION WITH AN ABSOLUTE VALUE EXCEEDING 120 KCAL/MOLE O2 (502 KJ/MOLE O2) AT 1000*C, ARE COMPLETELY CONTAINED IN THE FINELY COMMINUTED POWDER.
 2. A powder mixture according to claim 1, consisting of 80 to 99% of an atomized prealloyed powder of iron and 0.2 to 10% of said alloying elements, the oxides of which have a free energy of formation with an absolute value of less than 120 kcal/mole O2 (502 kJ/mole O2) at 1000* C, and of a finely comminuted powder consisting of a total of at least 25% and preferably at least 50% of said alloying elements, the oxides of which have a free energy of formation with an absolute value exceeding 120 kcal/mole O2 (502 kJ/mole O2) at 1000* C, at most 10% carbon, the rest iron and/or other alloying elements, the oxides of which have a free energy of formation with the absolute value lower than 120 kcal/mole O2 (502 kJ/mole O2) at 1000* C.
 3. A powder mixture according to claim 1, in which the alloying elements contained in the atomized prealloyed powder consist of one or more of the elements nickel, copper, molybdenum and cobalt.
 4. A powder mixture according to claim 1, in which the alloying elements of the finely comminuted powder are one or both of the elements chromium and manganese.
 5. A powder mixture according to claim 1, in which that the finely comminuted powder consists of a ferroalloy with a total of at least 25% and preferably at least 50% of said alloying elements, the oxides of which have a free energy of formation with an absolute value exceeding 120 kcal/mole O2 (502 kJ/mole O2) at 1000* C, at most 10% of carbon and the balance iron.
 6. A powder mixture according to claim 1, in which the finely comminuted powder has an average particle size according to Fischer of at most 20 Mu m and preferably at most 5 Mu m.
 7. A powder mixture according to claim 1, in which the particle size distribution of the atomized prealloyed powder is such that more than 90% and preferably more than 97% of the powder passes a sieve with the mesh opening of 175 Mu m.
 8. A powder mixture according to claim 1, in which the mixture contains in addition to the metal powder portion graphite and/or a lubricant such as zinc stearate up to a maximum 1% of each. 